1. Field of the Invention
The present invention relates to an ink jet recording device that uses heat energy for ejecting ink droplets toward a recording medium. The invention further relates to a method of driving the ink jet recording device.
2. Description of the Related Art
Japanese Laid-Open Patent Publication Nos. SHO-48-9622, SHO-54-51837, SHO-54-59936, SHO-54-161935 describe a type of ink jet recording device with channels filled with ink and nozzles, each in fluid communication with an ink channel. A pulse of heat is applied to the ink, which rapidly vaporizes as a result. The expansion of the resultant vapor bubble ejects a droplet of ink from the corresponding nozzle.
The most effective method of producing the heat pulse is with a thin film thermal resistor provided in the ink channel. Practical examples of thin film thermal resistors are described at page 58 of the "Nikkei Mechanical", published Dec. 28, 1992 and the "Hewlett-Packard Journal" published August 1988. These thermal resistor commonly include a thin film resistors with great thermal endurance, a metal thin film conductor, and a two-layer protective covering over the thin film resistor and the metal thin film conductor. The thin film forming the thin film resistors is about 0.1 .mu. thick. The two-layer structure of the protective covering is about 3 to 4 .mu. thick in total. The first layer of the protective covering is in contact with the thin film resistor and the metal thin film conductor and is for protection against oxidation and electrochemical corrosion. The second protective layer is provided for protecting the first protective layer against damage from cavitation.
Thermal resistors constructed as described above are used to pulse heat and rapidly vaporize the portion of the ink adjacent to the thermal resistor. Ink droplets are ejected by expansion of the resultant bubble. Printers must be able to rapidly repeat the ejection process which includes not only expansion of bubbles, but also the contraction and final disappearance of bubbles. Four conditions are required to produce a printer that can eject ink droplets stably and rapidly in succession at a high frequency.
The first condition relates to the generation of bubbles. Japanese Laid-Open Patent Publication Nos. SHO-55-27282 and SHO-56-27354 teach that in order to increase ejection efficiency, response, and frequency characteristics, the temperature at the surface of the thermal resistor must be rapidly increased to thereby invoke film boiling in the ink in contact with the thermal resistor, and the processes A through E shown in FIG. 1, which show the boiling characteristic curve of water, should be kept as short as possible. However, there are two points in the technical explanation and understanding in these publications which need correction.
The first point to be corrected is that the boiling characteristic curve shown in FIG. 1 represents a set stable state whereas ejection of ink droplets occurs in an unstable state. In the boiling characteristic curve shown in FIG. 1, the temperature at the heater surface that contacts the water is stable or rises and lowers slowly. Boiling which occurs from application of a pulse of heat is unsteady boiling. In fact, in subsequent research (see page 7 of Collection of Presentations from the 22nd Japan Thermal Transmission Symposium 1985-5), the inventors of the above-listed applications disclose that test bubbles were generated at 263.degree. C. This temperature matches the superheating limit of 270.degree. C. predicted by the theory of spontaneous nucleation. That is, bubbles are generated by unstable boiling, which is a very different phenomenon from the phenomenon of stable boiling represented in FIG. 1.
The second point to be corrected is the inappropriate use of the term film boiling. Film boiling assumes that conditions continue for a certain length of time. However, an extremely short pulse of heat rapidly generates a single bubble that vanishes in an extremely short period of time. In later research (see page 7 of the Collection of Presentations from the 22nd Japan Thermal Transmission Symposium 1985-5, on page 247 of the Collection of Presentations from the Journal for the 23rd Japan Thermal Transmission Symposium 1986-5, and on page 253 of the Collection of Presentations from the Journal for the 25th Japan Thermal Transmission Symposium 1988-6), the inventors of the above-listed applications changed their opinions to say that a small bubble is formed from spontaneous nucleation (also referred to as heterogenous nucleation) at a portion of the heater surface and afterward rapidly expands to the entire surface of the heater.
Therefore, it is technically incorrect to say that in order to increase ejection efficiency, response, and frequency characteristics, the temperature at the surface of the thermal resistor must be rapidly increased to thereby invoke film boiling in the ink in contact with the thermal resistor, and the processes A through E shown in FIG. 1, which shows the boiling characteristic curve of water, should be kept as short as possible. Taking the two points into consideration, a more accurate statement would be that the ink in contact with the surface of the heater should be brought into a film boiling condition in as short a time as possible.
Japanese Laid-Open Patent Publication No. HEI-03-266646 describes a thermal ink jet print head which uses a boiling phenomenon appearing when ink is heated under conditions different from those in the above-described research. The surface of the heater is raised at a speed of 10.sup.6 to 10.sup.9 .degree. C./S and the heat flux from the heater surface to the ink is set at 10.sup.7 to 10.sup.8 W/m.sup.2. The temperature at the heater surface and the ink adjacent to the heater surface is rapidly heated to the temperature at which homogeneous nucleation occurs. Ink is ejected by a homogeneous nucleated bubble.
The type of boiling that is ordinarily observed occurs by vapor nucleation. For example, vapor nucleation occurs at defects in the solid surface in contact with water when the temperature of the water reaches about 100.degree. C.
Spontaneous nucleation occurs when no defects are present in the solid surface in contact with the liquid to be boiled, that is, when the solid surface is perfectly uniform. Boiling activated by spontaneous nucleation occurs simultaneously over the entire boundary between the solid surface and the liquid. When the liquid to be boiled is water, boiling will start only when the temperature at the solid surface reaches about 270.degree. C. Spontaneous nucleation is also referred to as non-homogeneous nucleation because thus activated boiling occurs where solid and liquid coexist.
Homogeneous nucleation occurs only in superheated homogeneous liquids in contact with a uniform solid surface, as described above for spontaneous nucleation, that is rapidly heated. Refer to V. P Skripove, Metastable Liquids, John Wiley, New York 1974. The temperature at which homogeneous nucleation is assumed to occur in water is 312.5.degree. C. However, it is technically difficult to produce a heater which can generate the extremely rapid increase in temperature necessary for homogeneous nucleation to occur. In fact, there has been no confirmation of an actual heater with this capability.
Homogeneous nucleation is termed homogeneous, despite the presence of a solid surface, because homogeneous nucleation can be observed only in homogeneous liquids. Boiling begins in water adjacent to the boundary between the liquid and the solid surface when critical values for both the speed at which the solid surface rises and the heat flux that is transmitted to the liquid from the solid surface are exceeded and when the temperature at the solid surface and the water adjacent to the solid surface exceeds 312.5.degree. C.
Recently, Iida et al experimentally verified this phenomenon as discussed on page 334 of Collection of Presentations from the 27th Japan Thermal Transmission Symposium 1990-5. The invention described in Japanese Laid-Open Patent Publication No. HEI-03-266646 is based on the results of these experiments, in which the thermal resistor and the electrode are formed from the same material. However, the width of the electrode is at least five times and up to ten times the width of the thermal resister. This makes manufacturing an inexpensive large-scale line head difficult, although a head with a low density of 30 dpi could possibly be produced. That is, using this thermal resistor in a high density multi-nozzle type ink jet print head would be impossible without adding some further contrivance.
The second condition relates to the speed at which the thermal resistor is heated. Japanese Laid-Open Patent Publication No. SHO-55-161664 teaches that the average speed at which temperature of the thermal resistor increases (hereinafter referred to as "average speed of temperature increase") should be 1.times.10.sup.6 .degree. C./sec or more, preferably 3.times.10.sup.6 .degree. C./sec or more, and optimally 1.times.10.sup.7 .degree. C./sec or more. The liquid described in the publication is ink made mainly from ethanol. Recently, Iida et al performed precise experiments using pure ethanol. The average speed of temperature increase and the number of bubbles generated during these experiments are described in detail on page 712 of Collection of Presentations from the 28th Japan Thermal Transmission Symposium 1991-5. Although some discrepancies in the data can be accounted for by differences between pure ethanol and ink made mostly from ethanol, the most noteworthy result is that bubbles were generated at a density, which most closely governs ejection of ink, that was two orders of magnitude greater in ethanol than in water at the same average speed of temperature increase. That is, in order to generate the same number of bubbles in the same density, water must be heated at an average speed of temperature increase that is ten times faster than the average speed of temperature increase required for ethanol.
Therefore, a great technological leap is required to apply the invention described in Japanese Laid-Open Patent Publication No. SHO-55-161664 to water-based ink. An extremely fast average speed of temperature increase of about 1.times.10.sup.8 .degree.C./sec or more is required to stably eject water-based ink.
The average speed of temperature increase of 3.times.10.sup.7 .degree. C./sec could attained as reported on page 247 of the 23rd Japan Thermal Transmission Symposium Collection of Presentations 1986-6, and 7 to 8.times.10.sup.7 .degree. C./sec on the 25th Japan Thermal Transmission Symposium Collection of Presentations 1988-6. Further, the ink jet printers normally operate with 5 .mu.sec of heating pulse width. The thin film thermal resistor needs to be heated up to about 300.degree. C., so that the average speed of temperature increase in the ink jet printers is 300/(5.times.10.sup.-6)=6.times.10.sup.7 .degree. C./sec. Because the thin film thermal resistor used therein is covered with a protective layer of about 4 .mu.m on its surface, the speed of temperature increase on the surface of the thermal resistor contacting the ink would be slightly slower than the speed as calculated above.
When the average speed of temperature increase is further increased, it is confirmed that caviar-wise nucleation occurs in pure water as is theoretically predicted (See the 27th Japan Thermal Transmission Symposium Collection of Presentations 1990-5, page 334 and Presentation Papers published from Japan Mechanical Society, vol. B60, No. 572 (1994-4), page 264. In these reports, experiments were performed using a heater with no protection layer and the average speed of temperature increase of 9.3.times.10.sup.7 .degree. C./sec was attained.
Japanese Laid-Open Patent Publication No. HEI-3-266646 discloses that good ink ejections are performed when the average speed of temperature increase is in a range from 10.sup.6 to 10.sup.9 .degree. C./sec or more.
The third condition relates to the time between when the heat pulse starts and when the liquid starts to boil (hereinafter referred to as "the time to boiling start"). Asai et al discloses use of a naked heater without protective layers (page 7 of the Collection of Presentations from the 22nd Japan Thermal Transmission Symposium 1985-5). Although the lack of protective layers improves the rate of heat transmission, it also reduces reliability. Asai et al described tests using ethanol. Bubbles can be generated in ethanol at a temperature 70.degree. C. less than the temperature for generating bubbles in water. Asai et al used strobe techniques to observe the time between when a bubble was generated to when the bubble disappeared. Results of these observations are schematically shown in FIG. 2. Times listed indicate time elapsed after the initiation of a 10 .mu.S heat pulse. As can be seen, generation of the bubble begins 4 .mu.S after start of the thermal pulse. The bubble is at its maximum size at about 8 .mu.S after start of the thermal pulse. Afterward the bubble begins to contract. Secondary bubbles are generated after the first main bubble until the last secondary bubble completely vanishes at about 20 .mu.S after start of the heat pulse.
Asai et al describes using a heater similar to the above-described naked heater, but with a two-layer protective structure covering the alloy thin film resistor, in order to generate bubbles in water, which has nearly the same qualities as water-based ink (page 247 of Collection of Presentations from the 23rd Japan Thermal Transmission Symposium 1986-5). The results of the test are shown in FIG. 3. Power was applied so that the generation of a bubble begins at the declining edge of the thermal pulse (that is, when application of power is stopped). With this type of heater covered with the two-layer protective layer, 7 .mu.S was required from when generation of the bubble began to when the bubble reached its maximum size. This time is fixed and independent of the duration of the thermal pulse. No data was provided for time required for the bubble to disappear. However, because generation of secondary bubbles, which is a phenomenon similar to the bubble rebound phenomenon observed during cavitation, can also be observed when the pulse width of the thermal pulse is 10 .mu.S long, it can be assumed that bubbles begin to disappear about 25 to 30 .mu.S after start of bubble generation.
Asai et al discloses results of generating a bubble in actual water-based ink using a heater covered with the two-layered protective structure (page 253 of the Collection of Presentations from the 25th Japan Thermal Transmission Symposium 1988-6). Microscopic bubbles appeared at a portion of the heater surface at approximately 3 .mu.S after the start of the heat pulse. Afterward, a bubble was generated over the entire surface of the heater. Asai et al did not measure the temperature at the surface of the heater nor the heat flux to the liquid in tests of the third condition.
In contrast to this, Iida et al performed tests to accurately measure these values (see page 334 of Collection of Presentations from the 27th Japan Thermal Transmission Symposium 1990-5). Iida et al heated water using a heat pulse with duration of 5 .mu.S or more. Initial boiling nucleation in water was observed using a strobe light with an extremely short pulse of 10 nanoseconds. The shortest boiling start time was about 3.7 .mu.S. Theoretically predicted parameters of average speed of the temperature increase and the average speed of heat flux match with the conditions observed before and after the start of boiling. Two experiments and the results of the experiments are discussed below.
(1) In one experiment, heat was applied to 20.degree. C. water at an average speed of temperature increase of 0.56.times.10.sup.8 .degree. C./sec or greater and with an average heat flux of 1.5.times.10.sup.8 W/m.sup.2 or greater. The temperature at the surface of the heater at the start of boiling matched the theoretical temperature (312.5.degree. C.) at which homogeneous nucleation is believed to occur in water at atmospheric pressure. It was determined that boiling caused by this type of rapid heating is independent of the degree of liquid subcool (that is, the difference between the bulk temperature and the temperature at the surface of the heater when boiling starts).
(2) In another experiment, heat was applied at an average speed of temperature increase of 0.70.times.10.sup.8 .degree. C./sec or greater and with an average heat flux of 2.1.times.10.sup.8 W/m.sup.2 or greater, whereupon boiling caused by caviar-wise nucleation was observed for the first time in water. It should be noted that boiling did not occur by caviar-wise nucleation when; average speed of temperature increase or the average heat flux was less than these values. The characteristics of caviar-wise nucleation as observed in the above experiment are that first a multiplicity of small bubbles with a uniform size are generated across the entire surface of the heater at a uniform distribution. The number of bubbles rapidly increases. The bubbles couple to form a bubble film at the surface of the heater.
Contrarily, in normal homogeneous nucleation, small bubbles are generated erratically on the surface of the heater. The bubbles enlarge and couple to form the bubble film. The time period from nucleation to formation of the bubble film is much slower in normal homogeneous nucleation than in caviar-wise nucleation, which requires only 1 .mu.S or less. Although the time period from nucleation to formation of the bubble film has not been measured in spontaneous nucleation (heterogenous nucleation), considering that the speed of temperature rise and the heat flux are comparatively small values, the speed of formation is probably fairly slow.
In summary, the speed from the start of boiling to formation of a bubble film is slowest in spontaneous nucleation, faster in homogeneous nucleation, and fastest in caviar-wise nucleation. The shortest observed example of time from heat pulse to boiling is about 3 .mu.S. This can be estimated as the limit for conventional thermal resistors which require a thick two-layer protective covering.
The fourth condition for allowing stable ejection of ink at a high repetition speed relates to the contraction and disappearance of bubbles. There have been many attempts to control the speed at which bubbles contract and disappear in order to smooth recuperation of the meniscus after ejection and moreover to shorten the frequency and increase the speed of ejections. For example, Japanese Laid-Open Patent Publication No. SHO-55-132267 describes setting the duration of time required for the surface of the heater to cool to longer than the time required to heat the surface of the heater. Japanese Laid-Open Patent Publication Nos. SHO-55-161662, SHO-55-161663, and SHO-56-13177 describe setting the time required for the temperature at the surface of the heater to cool by half to a duration of time longer than the time required to heat the surface but shorter than four times the time required to heat the surface. However these publications do not accurately disclose data or the technical basis for these determinations. Additionally, the technical content and results of controlling the speed of bubble contraction and disappearance is questionable.
Publications by Asai and others refute these inventions (Collection of Presentations from the 22nd Japan Thermal Transmission Symposium 1985-5 and in Collection of Presentations from the 23rd Japan Thermal Transmission Symposium 1986-5). A film shaped bubble generated on the heater by application of a pulse of heat expands explosively at high pressure (several tens to hundreds of atmospheres) and at high temperature (about 300.degree. C.). Expanding gas in the bubble is cooled by the surrounding room temperature liquid, i.e., the ink. When the bubble is at its maximum size, the interior of the bubble is almost a complete vacuum. In the next instant, the bubble begins to contract, and vanishes in about 5 .mu.S. The heat flux from the surface of the heater to the bubble is negligible when the heater is covered by the bubble. Therefore, the speed of contraction is virtually constant and independent of the temperature at the surface of the heater.
However, when the temperature at the surface of the heater does not decrease even after the initial bubble disappears, secondary bubbles are repeatedly generated. Generation of secondary bubbles interferes with recuperation of the meniscus after ink is ejected. Inducing boiling by heating a portion of a liquid that is cooler than boiling temperature is termed subcool boiling. Thermal ink jet print heads use subcool boiling when the amount of subcooling is large. As can be seen in FIG. 3, the time required for a bubble to contract and disappear is twice as long as the time required to generate the bubble. Before a bubble is generated, a pulse of heat with long duration (10 to 50 .mu.S) is applied to heat the water on the heater, to increase the volume of water that boils as a result, and to increase the volume of the bubble. The time for contraction of the resultant large volume bubble is about 10 .mu.S. Whether the secondary generation of bubbles shown in FIG. 3 results from insufficient cooling of the heater temperature or from cavitation by the contraction of the bubble volume is unknown, but secondary generation of bubbles occurs in all bubble contractions in conventional technology.
In Japanese Laid-Open Patent Publication Nos. SHO-55-27281 and SHO-55-27282, Asai et al teaches that the rise in temperature of the heater and the subsequent cooling speed should be as rapid as possible. The only fixed quantity mentioned however is an extremely long pulse of 100 .mu.S.
In order to increase the frequency or ejections and provide stable ejection at the same time, boiling must be started as quickly as possible after application of the energy pulse to the thermal resistor and also the expanded bubble must be caused to disappear as rapidly as possible. Conventional technology requires that thin film resistors include a two-layer protective coating. Such thin film resistors require at least 3 .mu.S from after start of application of the energy pulse to when the film boiling begins. Even naked thin film thermal resistors with no protective layers, which are unreliable and impractical, require at least 4 .mu.S to generate bubbles in ethanol. Bubbles require 30 .mu.S or more to disappear from start of the pulse application with thin film thermal resistors with two-layer protection coverings. Bubbles generated by naked thermal resistors in ethanol require 20 .mu.S or more to disappear. Secondary bubbles are also always generated. Secondary generation of bubbles increases the time required for bubbles to disappear, thereby interfering with efforts to increase the frequency of ejections. A large amount of energy, that is, about 17 .mu.J/50.times.50 .mu.m.sup.2 or more, is required to start boiling with film thermal resistors with two-layer protective coverings. Although details will be explained later in the embodiment of this application, only several .mu.J/50.times.50 .mu.m or less of energy are required to start boiling by a protection-layerless thin film thermal resistor. Therefore, almost all of the energy applied to conventional heaters is used to heat the substrate. For this reason, the surface of the heater is hot while the bubble is vanishing. This is a major source of secondary bubble generation. Heating of the substrate is brought about by the material from which the ink channel is produced and the temperature of the ink. This is a source of unstable ink ejection.
Referring back to the second condition relating to the speed at which the thermal resistor is heated, it is technically difficult to increase the average speed of temperature increase. In fact, there is few reliable experimental reports on the average speed of temperature increase of more than 1.times.10.sup.8 .degree. C./sec. Japan Hardcopy '94 Presentation, 1994-6, page 141 is one example of the report. Nevertheless, it has been considered that the faster the speed at which the thermal resistor is heated, the more effective in performing ink ejection.
In order to increase the average speed of temperature increase, it is essential to employ a heater in which a protective layer is not provided on the surface of the thin film thermal resistor. Even if the protective layer is provided thereon, its thickness must be as thin as possible, that is, about 100 .ANG..